The invention relates to a device and a method for charging or charge reversing an aerosol into a defined charge state of a bipolar diffusion charging (e.g. symmetrical or equilibrium charge distribution according to Fuchs, N., On the Stationary Charge Distribution on Aerosol Particles in a Bipolar Ionic Atmosphere, Geofis. Pura Appl., Vol. 56, 1963, pp. 185–192) with the aid of an electrical discharge in the aerosol space.
Alternatively, the device and method are suitable for setting a defined unipolar charge state of the aerosol.
Technical aerosols in industry and research often exhibit a medium to high electrical charge. Neutralization enables the production of aerosols of a defined charge state. Above all in research and in aerosol measurement technology involving instruments such as a differential mobility analyzer (DMA), neutralization can be an indispensable prerequisite. In addition, with the aid of neutralization the probability of electrical discharges or dust explosions is reduced, and any tendency for particle deposition in pipes and equipment parts is counteracted.
Known methods for aerosol neutralization employ radioactive sources or corona discharge sources.
Radioactive sources, by virtue of radioactive decay, produce ionizing radiation which produces equal quantities of anions and cations in the aerosol space. The gas ions subsequently charge or reverse the charge as the case may be, altering the aerosol into the theoretically describable charge state of the bipolar diffusion charge (cf. Fuchs, N., On the Stationary Charge Distribution on Aerosol Particles in a Bipolar Ionic Atmosphere, Geofis. Pura Appl., Vol. 56, 1963, pp. 185–192).
The application of radioactive sources, aside from safety concerns, is very simple. In the case of a suitable arrangement, an adjustment or readjustment need not be carried out. To be sure, the application field of radioactive sources is limited by several disadvantages:                The safety requirements concerning the radioactive source are high.        The neutralization into the equilibrium state (as described in Fuchs) is practical only for small aerosol volume streams (<150 l/min), low aerosol concentrations, and low initial charges.        The costs are very high.        
Neutralizations on the basis of the corona discharge are in principle capable of handling greater aerosol volume streams, higher aerosol concentrations, and higher initial charges of the aerosol.
Romay et al. (Romay, F., Liu, B., Pui, D., A Sonic Jet Corona Ionizer for Electrostatic Discharge and Aerosol Neutralization, Aerosol Sci. Tech., Vol. 20, 1994, pp. 31–41) speaks of three problems in neutralization with corona discharges:                Unequal production rates of positive and negative ions.        Ozone generation.        Particle production through sputtering or chemical reactions.        
Previous devices have avoided producing the corona discharge in the aerosol space itself. An electric field required to produce the corona discharge in the aerosol space causes partial precipitation of the aerosol, and the particles are not charged into the desired charge state of diffusion-based bipolar charging.
This problem can be solved through producing the necessary ions of both polarities in one or several separate process spaces. Then, with the aid of a particle-free carrier gas, the ions are introduced into the field-free aerosol space (e.g. Romay, F., Liu, B., Pui, D., A Sonic Jet Corona Ionizer for Electrostatic Discharge and Aerosol Neutralization, Aerosol Sci. Tech., Vol. 20, 1994, pp. 31–41; Zamorani, E., Ottobrini, G., Aerosol Particle Neutralization to Boltzmann's Equilibrium by AC Corona Discharge, J. Aerosol Sci., Vol. 9, pp. 31–39; Adachi, M., Pui, D., Liu, B., Aerosol Charge Neutralization by a Corona Ionizer, Aerosol Sci. Tech., Vol. 18, 1993, pp. 48–58). This dilutes the aerosol. In addition, most of the gas ions are deposited onto the walls or are lost through recombination. The resulting need to overproduce the gas ions increases the ozone yield.
Devices that operate directly in the aerosol space with electric discharges were developed by Hinds, W., Kennedy, N., An Ion Generator for Neutralizing Concentrated Aerosols, Aerosol Sci. Tech., Vol. 22, 2000, pp. 214–220 and Gutsch, A., Agglomeration feinster gasgetragener Partikel unter dem Einfluss elektrischer Kräfte [Agglomeration of Superfine Gas-borne Particles under the Influence of Electrical Forces], Dissertation, University Fridericiana Karlsruhe, 1995.
Gutsch uses an arrangement with two points opposite each other in a channel that accommodates an aerosol flow. A constant positive or negative high voltage, as the case may be, is temporarily applied to each of the two points. A bipolar corona discharge is generated between the two points. Both points act as active electrodes and produce positive or negative gas ions, as the case may be.
Hinds developed an apparatus with a total of five electrodes, including a central electrode and four points aligned axially in the flow in a 90° arrangement. The four points are biased to the same potential, while the axial electrode forms the antipole (in this case positive). Due to smaller curvature radii of the four electrodes, more negative than positive charges develop. The precise ratio of the positive and negative charge magnitudes is controlled through the electrode radii and the voltage.
However, the methods using discharging in the aerosol space achieve only a charge reduction (Hinds) or charging to an undefined bipolar charge state (Gutsch). Neither device can be shown to charge or reverse charge the aerosol into the diffusion-based bipolar charge distribution. In addition, considerable deposition occurs.
The object of the invention is to create a method whereby gas ions are produced directly in the aerosol space with the aid of electric discharges such that the aerosol attains the diffusion-based, bipolar charge distribution. The device for this purpose should favorably realize the advantages of the described method. Diffusion separation or separation through electrical forces should be avoided to the extent possible, and charging into the diffusion-based equilibrium state should occur despite the presence of an electric field.